Macromolecules

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Discussion Questions

What are macromolecules?
What are some examples of inorganic macromolecules?
How is the properties of sulfur related to its structure?

A macromolecule is a very large molecule having a polymeric chain structure. Proteins, polysaccharides, genes, ruber, and synthetic polymers consist of macromolecules. For synthetic polymers, here are the abbreviations for some common polymers:

HDPE: high density polyethylene
LDPE: low density polyethylene
PET: polyethylene terephthalate
PP: polypropylene
PS: polystyrene
PVA: polyvinyl alcohol
PVC: polyvinyl chloride

There are only a few known inorganic macromolecules. For example, when liquid sulfur is poured into cold water, long chains of ...-S-S-S-S-S-... are formed. These molecules are present in a phase known as elastic sulfur. Inorganic macromolecules can be divided into several categories: solids formed mainly due to covalent bonds, organosilanes, siloxanes and organosiloxanes.

Solids formed mainly due to covalent bonds

We have mentioned diamond, graphite, silicon, germanium, etc as large molecules. In this section, some more examples are given: Zinc sulfide has two forms (or phases): the wurtzite and the zinc blende. These are typical structures, because many other common compounds have the same structure. Knowing the bonding, geometry, symmetry of these structures is important, because scientific discussion and applications are based on their structures. The difference between wurtzite and zinc blende illustrates some fundamental geometry and symmetry difference.

Wurtzite is a typical mineral, often involving iron and zinc sulfide, and formulated as (Fe,Zn)S For example: ZnO, SiC, AlN, CaSe, BN, C(Hexagonal Diamond) all have the same crystal structure as wurtzite in terms of bonding, symmetry, packing sequence etc.

The zinc blende is cubic. It can be perceived as a f.c.c. lattice of S with half of the tetrahedral sites occupied by Zn. A unit cell of the crystal structure is shown in Zinc blende, and you can see this structure from various perspective. The diagram on the page can be manipulated by moving the mouse. The same group has also got a wurtzite structure that you can manipulate. By the way, the zinc blende structure has the same bonding skeleton as the diamond structure. Thus, the wurtzite and zinc blende structures are two typical structural types for inorganic macromolecules.

Silanes and Organosilanes

Since Si-Si is much weaker than C-C bonds, silanes Si_nH_2n₊₂, are not as stable as alkanes. Methane is a stable gas, but silane SiH₄ explode as soon as it comes in contact with air. Silane is made by reacting Mg₂Si with dilute HCl, but the silane produced burns as soon as it comes in contact with air at the surface of the solution:

\[Mg_2Si + 4 HCl \rightarrow 2 MgCl_2 + SiH_4 \nonumber \]

\[SiH_4 + 2 O_2 \rightarrow SiO_2 + 2 H_2O \nonumber \]

When an organosilane reacts with water, the Si-Si bonds break:

\[R_3Si-SiR_3 + 2 H_2O \rightarrow 2 R_3SiOH + H_2 \nonumber \]

where \(R\) is an alkyl or aryl group. Organosilanes have some unique applications, and silane coupling agents, R_nSiX₄_-n (X being a halogen) are useful.

Siloxanes and Organosiloxanes

Siloxanes and organosiloxanes have Si-O-Si-O-Si linkage, and these are stable polymers due to the strong Si-O-Si bonds. Since Si atoms tend to form four bonds, these polymers form two- and three-dimensional networks, making them excellent sealants.

   R         R
   |         |
- Si-O-Si-O-Si-R
   |    |    |
   R    O    O
        |    |
     R-Si-O-Si-O
        |    |

Silicates are based on Si-O-Si linkages. Quartz for example is based on three-dimensional frame work of these linkages.

How is the properties of sulfur related to its structure?

Sulfur is in the same group as oxygen, and its valence electrons have the electron configuration:

3s²3p⁴.

These six electrons usually occupy the four sp³ hybrid orbitals, two of which have a pair of electrons each, and the other two have only one electron each. Thus, sulfur usually form two bonds such as H₂S, its structure similar to H₂O. Sulfur atoms bond to each other forming the cyclic molecules such as S₆ and S₈. The two bonds and the two lone pairs of electrons around the sulfur point to the direction of a tetrahedron, making the -S- angle approximately 100 degrees.

Sulfur has three allotropes: rhombic, monoclinic, and plastic sulfur. At room temperature, monoclinic sulfur is the the stable form. When heated, monoclinic sulfur melts to form a viscous liquid at 119 degree C at the atmosphere pressure. At higher pressure, the monoclinic sulfur transforms into the rhombic sulfur. Both crystalline forms have the S₈ crown shaped molecule and the plastic sulfur has a chain structure of unspecified number of atoms S_n (n is a very large unspecified number).

Since we are talking about the element sulfur, we might include some information on it. Sulfur occur as minerals: pyrite, FeS₂ (also known as fool's gold); gypsum, CaSO₄ºH₂O, (when dehydrated it is called paster of paris). Sulfur also occur as an element in nature because some bacterial converts sulfur oxides and other compounds to elemental sulfur. Elemental sulfur is extrated from under ground by hot water and air in a process called the Frasch process.

Sulfur is easily oxidized to sulfur dioxide

\[S_8 + 8 O_2 \rightarrow 8 SO_2 \nonumber \]

The SO₂ is a bent molecule with an O-S-O angle of 120 degrees. It reacts further with water and with oxygen as examplified by the following reaction:

SO₂ + H₂O = H₂SO₃ (sulfurous acid)
2 SO₂ + O₂ = 2 SO₃ (catalyst required)
SO₃ + H₂O = H₂SO₄ (sulfuric acid)
Na₂SO₃ + S = Na₂S(=S)O₃ (known as hypo)

What are the properties and structures of phosphorus?

Phosphorus has only 5 valence elections, one less electron than sulfur. Thus it tends to form three bonds around a P atom. The analogous of ammonia is phosphine, PH₃, which is air stable with a melting point of -88^o C. White phosphorus can be obtained by reducing phosphorus oxide with carbon:

\[P_4O_{10} + 10 C \rightarrow P_4 + 10 CO \nonumber \]

The crystals consist of P₄ molecules as shown here, and it undergoes natural combustion if not stored under water. The combustion leads to the formation of P₄O₁₀. White phosphorus converts to stable red, black, violet and scarlet phosphorus, which have complicated network of macromolecules. The bonding in P₄ can be explained in the same manner as that described for sulfur, but that is left as an exercise. As most other non-metallic elements, phosphorus also form a complicated covalent solid.

Contributors and Attributions

Chung (Peter) Chieh (Professor Emeritus, Chemistry @ University of Waterloo)